1. Field of the Invention
The present invention relates to centrifuges and more particularly to an improved system for monitoring the actual speed and identifying maximum safe speed rating of a centrifuge rotor.
2. Description of Related Art
A centrifuge operation presents a unique set of design criteria where precision control of the rotational operation of the centrifuge is required. The wide variety of biological and chemical experimental research which use centrifugation as their primary tool to achieve component separation and perform experimental assays places a requirement of versatility on the operational characteristics which must be built into the centrifuge. At the same time, safety concerns have to be addressed.
The centrifuge rotor is driven to extremely high rotational speeds in order to generate the centrifugal field required for biological research use. The high rotational speeds of the rotor cause a severe build up of kinetic energy during operation, which if released (as when the rotor breaks into pieces while in rotation), can lead to destruction of the centrifuge and injury or damage to its surrounding environment as well as the human operator. Centrifuge rotors will fail if subject to excess stress under the high centrifuge field when the rotor is run in excess of the speed designed for its safe operation.
In order to make it possible to perform a variety of different kinds of separations, many centrifuges are designed so that they can operate with any of a variety of different kinds and sizes of rotors. The rotors can be interchangeably used in conjunction with the same centrifuge motor and drive shaft, each rotor having a different weight and strength of material and a different maximum safe speed above which the particular rotor should not be operated. Because failure of any rotor can be catastrophic, it is important that the centrifuges be able to determine the maximum safe speed of a rotor without having to rely upon the attentiveness of its operator.
Accurate control of the speed of a rotor also makes it important that a centrifuge include an accurate tachometer for generating a signal indicative of the actual speed of the rotor.
It is therefore clear that a versatile centrifugation system requires in part: (1) a maximum safe rotor speed be identified for each rotor; and (2) the operation of the rotor during centrifugation be monitored and controlled. As a result, some centrifuges are equipped with detection circuits to achieve these objectives. One such system is disclosed in U.S. Pat. No. 4,551,715 commonly assigned to the assignee of the present invention, which is hereby incorporated by reference. In the disclosed specification, a method of rotor identification and determination of the rotor's maximum safe speed is presented which relies on the detection of changing magnetic flux from magnetic coding elements to provide the necessary rotor identification and maximum safe speed information as well as actual rotor speed. Referring to FIGS. 1A and B, a single set of magnetic coding elements, e.g. permanent magnets 24 are imbedded in a circular array in the base 12 of the rotor 10. The permutation of the magnetic orientation of the magnets 14 is unique to the rotor model and provides positive identification of the rotor model. The transducer 16 is a Hall effect sensor which is used to detect the magnetic orientation of the permanent magnets 14. Magnets are also imbedded in the base of each model of interchangeable rotor designed for use with the centrifuge.
Specifically six magnets 14 are spaced at equal intervals in a circle and each is positioned to direct either a north-oriented or south-oriented magnetic field outward from the base 12 of the rotor 14 for detection by the Hall effect sensor 16. The sensor 16 detects a changing magnetic reluctance as the permanent magnets 14 rotate past the fixed sensor and induce a voltage in the sensor. A series of sharply defined voltage pulses of positive and negative polarity corresponding to north and south magnetic orientations, respectively, are generated by the sensor 16 and amplified in the detection circuit (not shown). The pulses represent the model of rotor used. Stored in the central processing unit (not shown) is an information listing identifying the maximum rated speed for each model of rotor. Once the rotor is identified on the basis of the pulses, the central processing unit reads the maximum speed rating information stored within its memory. The maximum permitted operation speed of the centrifuge is then set not to exceed the rated speed of the rotor. Thus the patent discloses an embodiment which is able to identify a rotor on the basis of a single transducer according to the combination of the north and south-oriented magnets 14 and the order that they pass the hall effect sensor 16.
The actual rotor speed can also be determined from the counting of the voltage pulses. For overspeed protection, the central processing unit is used to compare actual rotor speed with the maximum speed rating of the rotor. The central processing unit also is aware of what had been programmed at the operator keyboard for the desired acceleration and speed. The central processing unit functions to prevent the rotor from being actually operated beyond its intended rating even if a higher speed has been programmed.
As explained in the patent, the use of the coding scheme with a six-magnet array allows the detection circuitry to distinguish up to eleven different kinds of rotors. Stated differently, the coding scheme allows as many as eleven kinds of rotors, each with a different respective maximum safe speed, to be used with a particular centrifuge which incorporates the disclosed rotor identification technique. With the advent of new generation ultracentrifuges, additional rotors are designed for higher speed operations. It follows that the new ultracentrifuges will be able to accommodate rotors of higher speed ratings in addition to speed ratings of the eleven lower speed rotors. It is therefore desirable to design a system of rotor identification in new generation ultracentrifuges which will operate with a larger selection of rotors. It is also desirable to design the system to be compatible with prior art centrifuges and rotors.